Or maybe that’s giving Tolkien a lot of credit. Might be that he’s more interested in the imagery and situations and doesn’t feel like it’s that important to get into the why. And I agree with him! Overly fussy operational nitpicking is the death of drama and a classic nerd hangup.

You may have read my comment in the op as critical but I assure you it was just an observation about his style. And a rationale,I guess, that it’s okay for there to be unexplained stuff like unaccounted treasure hoards stuck in Mirkwood.